The proprietors of copyrights in pre-recorded video cassettes and their publishers have taken a number of measures to discourage and prevent the unauthorized duplication of such cassettes. In typical unilateral copy prevention techniques, the video signal being transferred to a pre-recorded tape is modified in such a manner that, at least theoretically, the modified signal will play properly on a television set during normal playback on a video cassette recorder (VCR) but will disturb the operation of a duplicating VCR in such a manner that the unauthorized tape copy will not play back properly. In unilateral copy protection schemes, only the signal on the pre-recorded tape is modified. Bilateral schemes have also been proposed in which a signal incorporated during the pre-recording process cooperates with copy prevention circuitry in a VCR. The present invention is only concerned with certain unilateral copy prevention techniques.
In the most common current copy prevention schemes, the video signal is modified during the vertical blanking interval. In one such technique, the pulse spacing in the serrated vertical sync interval is altered in such a way that when the serrated vertical sync signal portion is integrated, as is commonly done to recover the vertical field synchronization information, the output level is less than if a standard pulse spacing were present. In theory, the modified integrated vertical sync level is high enough to avoid visible effects on a "standard" television set, but not high enough to properly synchronize the helical scan drum on a VCR which transports the tape past the recording head. The usual result is that when a modified prerecorded tape is duplicated and the duplicate tape played, the image contains vertically moving noise bands.
In another such copy prevention technique, the video signal is modified to place spurious sync pulses in the vertical blanking interval, each such pulse immediately followed by a pulse having a peak white level or at least a "gray" level substantially "whiter" than a reference blanking level. The intended effect is to cause misadjustment of an automatic gain control (AGC) circuit in the copying VCR which sets the amplitude of the video signal being recorded by reference to an expected blanking level immediately following a pulse having a standard sync tip level. If the AGC circuit references to a white level or a level more positive (conventionally) than a standard blanking level, the AGC circuit adjusts the video amplitude in a diminished direction during recording. When the tape is subsequently played, the brightness is correspondingly diminished. If the modification is constant in amplitude and period, the viewer could manually adjust the brightness and contrast controls of the television set to compensate somewhat for the diminished video signal amplitude, although an overall loss in picture contrast might be uncorrectable. However, the insertion of gray and peak white pulses is often done on a random basis to prevent such a simple correction.
The above described copy prevention techniques are premised on the assumption that the video signal modifications will adversely affect the recording of video signals so modified but will not have any adverse effects on the playing of the modified signal on "standard" television sets. While television sets are designed to accommodate video signals adhering to certain standards, there are variations in the tolerances by which television sets are manufactured and in the picture controls which are provided on any given model.
In practice, it has been found that some television sets experience adverse image effects during the playback of original pre-recorded tapes incorporating such modifications. When the tapes have been modified by the vertical sync interval modification, the image effect on such television sets is an inability to synchronize vertically or vertical "rolling". This effect can be overcome by adjustment of the vertical hold control on the set. However, many current television sets perform the vertical hold function automatically and do not provide for manual vertical hold adjustment. With the insertion of gray and white pulses, some televisions adjust their AGC's by referencing to these pulses which causes a fading and blasting of image brightness level. When the pulses occur on a random time and level basis, there is no control on conventional television sets which can correct the effect.
It is emphasized that these adverse effects occur with some, though not all, television sets connected to VCR's and during the playback of the modified tapes in the manner intended by the publisher. That is, a viewer has paid to purchase or rent a tape which is virtually useless because of incompatabilities between his particular combination of video equipment and the video signal modification on the tape. While the viewer can often get a reimbursement from the tape vendor, inconveniences are suffered by the viewer and the vendor. Further, it is sometimes impossible for a viewer to find operable pre-recorded tapes of desired movies.